First, you need to log in to your Yahoo account.
Then, on the top corner of your Yahoo Mail window, you will see this: Hi, YOURNAME
... (YOURNAME actually is your actual name on Yahoo)
Right next to your name is a drop down arrow, click on that and select Account Info
Another Yahoo window will open asking for you to log in again. Just type your password to log in. Don't worry, it's just one of the security checks of Yahoo before editing the information for your account.
After you successfully logged in, You will be taken to your Yahoo Account Info page. And just below that title, and at the left of your name, you will see "Edit profile details". Click on that.
On the next page, click the topmost Edit button to change your last name.
Change your last name then click save.
Please read some of my iPhone / iPod touch tips like jailbreaking and unlocking, click here.If you need online help about jailbreaking and unlocking, you can contact me personally at this email: firstname.lastname@example.org...